Calcination method and apparatus



Sept. 24, 1968 c. F. VON DREUSCHE, JR.. ET AL 3,402,920

CALCI NATION METHOD AND APPARATUS Filed Feb. 10, 1967 4 Sheets-Sheet 1 W 2o E 2 50 H I 1 1 III 39 1 l I l I l l l 1 n II I I In 30 1 I III I -53\\ u 30 l I I5 22 29 22 45 In 29 I I 30 4I q I 42 INVENTORS 1 CHARLES E vonDREUSCHE, IR. 1 1 flROBERT E. SULLIVAN Sept. 24, 1968 c, VON DREUSCHE, JR ET AL 3,402,920

CALCINATION METHOD AND APPARATUS 4 Sheets-Sheet 2 Filed Feb. 10. 1967 I 'll III N oE BY M, 7/M,/;

e/d/M ATTORNEYS Sept. 24, 1968 c. F. VON DREUSCHE, JR.. ET AL 3,402,920

CALCINATION METHOD AND APPARATUS Filed Feb. 10, 1967 4 Sheets-Sheet 5 I I I I [III/III! INVENTORS CHARLES F. vonDREUSCHE, 31?.

ROBERT E. SULLIVAN ATTORNEYS Sept. 24, 1968 c. F. VON DREUSCHE, JR.. ET 3,402,920

CALCINATION METHOD AND APPARATUS 4 Sheets-Sheet 4 Filed Feb. 1O, 1967 FIG. 5

INVENTO S CHARLES E von DREUSCHEFR ROBERT E.

A BY

ZM/M

SULLIVAN ATTORNEYS United States Patent 3,402,920 CALCINATION METHOD AND APPARATUS Charles F. Von Dreusche, Jr., Ramsey, N.J., and Robert E. Sullivan, Anaconda, Deer Lodge, Mont., assignors to The Anaconda Company, New York, N.Y., a corporation of Montana Filed Feb. 10, 1967, Ser. No. 615,148 15 Claims. (Cl. 263-26) ABSTRACT OF THE DISCLOSURE The metal parts of apparatus adapted to calcine solid materials in an atmosphere containing relatively high concentrations of such corrosive gases as hydrogen chloride and the like are protected against attack by the hot corrosive furnace gases by providing cooling and special insulating means for maintaining these metal parts at a temperature above the dew point of hydrochloric acid and below about 500 F. when the furnace gases themselves are at a temperature of between about 1000 F. and 1800 F.

This invention relates to improvements in method and apparatus for calcining solid materials in an atmosphere containing relatively high concentrations of such corrosive gases as hydrogen chloride and the like.

There are a number of chemical and metallurgical processes which involve the heating or treatment of solid materials in the presence of hot furnace gases. Such processes can be carried out in a variety of conventional furnaces such, for example, as a rotary kiln or a multiple hearth furnace of the Herreshotf type. However, conventional furnace construction involves the use of numerous metal parts which are subject to corrosion if exposed to hot furnace gases that are corrosive in nature, as is the case when these gases contain hydrogen chloride, chlorine, or similar corrosive gases. For example, cast iron furnace parts will corrode away at the rate of 0.1 inch per month when exposed to hydrogen chloride gas at a temperature of 950 F. and .at the rate of 0.005 inch per month when exposed to such gas at 500 F., and pure nickel corrodes away at the same rates when exposed to hydr-ogen chloride gas at a temperature of 1300 F. and 950 F., respectively. Carbon steel and all sorts of stainless steel and other corrosion-resistant alloys have corrosion-resistant properties that lie somewhere between cast iron and pure nickel. As a result, it has heretofore been found that the temperature of corrosive furnace gases that may be contained within a furnace of conventional construction ordinarily should not be allowed to exceed about 950 F., and preferably should not exceed about 500 F., in order to prevent excessive corrosion of the metal furnace parts exposed to the corrosive atmosphere. This limitation on the temperature of corrosive furnace gases that may be contained within the furnace severely limits the usefulness of conventional furnaces for use in many chemical and metallurgical processes of the type referred to.

By way of example, it has heretofore been proposed that aluminum oxide or aluminum suitable for use as the feed material in the electrolytic production of metallic aluminum be produced by digesting aluminum-containing clay with hydrochloric acid to obtain a digestion liquor from which relatively pure crystals of aluminum hexahydrate are separated and recovered. The aluminum chloride hexahydrate crystals are then subjected to a high temperature calcination operation to drive olf the water of hydration and the chlorine content thereof in the form of water vapor and hydrogen chloride gas and thereby obtain the desired aluminum oxide feed material. In order to produce a pot line grade aluminum oxide product, and

in order for the process to be economically feasible, it is important that substantially all of the chlorine content of the aluminum chloride hexahydrate crystals be driven therefrom and recovered (as hydrogen chloride) for reuse in the process. To achieve this result we have" found that the aluminum chloride hexahydrate precipitate must first be heated to at least about 700 F. to drive off the bulk of the water of hydration and hydrogen chloride, and that the aluminum oxide product must then be calcined at a temperature of about 1600 or 1700" F. (and in some cases at a temperature of at least about 2'100 F.) to obtain pot line grade alumina of the requisite purity. This means that the furnace gases contain relatively high concentrations of corrosive hydrogen chloride gas and that the temperature of the furnace gases ranges from about 1000 F. to at least about 1 800 F. However, as previously indicated the extremely corrosive character of the gases evolved during the decomposition of aluminum chloride hexahydrate and the unusually high temperature at which the calcination operation must be carried out have made it impossible or at least economically impractical to carry out the calcination operation in conventional calcination apparatus.

Summary 0 the invention We have made an intensive investigation of the problems attending the high temperature decomposition and calcination of aluminum chloride hexahydrate and similar materials, and we have discovered that the key to successful furnace operation lies in careful control of the temperature of the metal parts of the furnace that are in contact with the hot corrosive furnace gases. Specifically, we have found that the metal parts of the furnace exposed to the corrosive furnace gases can be protected against attack by these gases by insulating and cooling these parts of the furnace with the special insulating and cooling means of our invention so that said metal parts are maintained at a temperature above the dew point of hydrochloric acid and below about 500 F. when said furnace gases are themselves at a temperature of between about 1000 F. and 1800 F.

The high temperature decomposition and calcination operations to which our invention applies can be carried out in a variety of apparatus. However, we have found that the heating of solid materials under the extreme conditions encountered in the practice of our invention can best be carried out in apparatus comprising :a refractory-lined metal furnace shell having at least one annular hearth disposed therein, the furnace being provided with a vertically disposed rotatable center shaft extending upwardly through the center opening of the annular hearth and with a plurality of radially extending rabble arms secured at their inner ends to the rotatable center shaft above the annular hearth. In the preferred furnace construction of our invention the rotatable center shaft comprises a hollow metal core that is provided with cooling fluid passageways for the circulation of a cooling fluid therethrough, the metal core being covered with an inner layer of a relatively soft insulating material that is immune to attack by hot hydrogen chloride-containing gases at a temperature of at least 1800 -F. and with an outer layer of a preformed structural refractory material that also is resistant to attack by hot hydrogen chloride gas and that protects the underlying 'layer of insulating material from physical attrition. The radial rabble arms each comprises an essentially hollow metal core provided with cooling fluid passageways which communicate with the cooling fluid passageways in the hollow metal core of the rotatable center shaft, the metal core of the rabble arm being covered with an inner layer of relatively soft fibrous insulating material that is immune to attack by hydrogen chloride-containing gases at a temperature of at least about 1800 F. and with an outer layer of preformed structural refractory material that also is resistant to attack by hot hydrogen chloride gas. The outer layer of refractory material comprises a generally semi-cylindrical upper section that fits over the inner layer of fibrous insulating material and a generally planar lower section having integrally formed rabble teeth depending therefrom. The inner layer of relatively soft insulating material disposed between the metal core and the outer layer of preformed refractory material is subjected to compressive forces only whereby damage to said layer of relatively soft insulating material as a result of the tangential forces exerted against the dependent rabble teeth is avoided.

In the preferred embodiment of our invention, the relatively soft unconsolidated insulating material is selected from the group consisting of micro-quartz, silica wool, low alkali glass wool and alumino-silicate refractory wool, and the structural refractory material is selected from the group consisting of firebrick, a silica based refractory containing at least about 90% SiO and an alumina based refractory containing at least about 90% A1 Moreover, the generally planar lower section of the outer layer of structural refractory material that covers each radial rabble arm is advantageously provided with a pair of inwardly turned longitudinal lip members which are supported by longitudinal flange members formed along the bottom longitudinal edges of the hollow metal core of the rabble arm, the adjacent surfaces of said generally planar lower section and the metal core of the rabble arm being separated by an inner layer of said relatively soft unconsolidated insulating material.

The improved furnace construction of our invention may be employed in conjunction with a variety of conventional calcination apparatus or furnaces to protect the metal parts of such furnaces against attach by hot corrosive furnace gases. In particular, we have found our invention to be useful to protect the metal parts of a multiple hearth furnace of the Herreshofi type against attack by hydrogen chloride-containing furnace gases produced by the decomposition of aluminum chloride hexahydrate and by the calcination of the aluminum oxide obtained as a result thereof. However, it will readily be appreciated that our improvement in furnace construction is not limited to multiple "hearth furnaces or to the decomposition or calcination of chloride-containing solid materials.

Description of drawings Our improvments in furnace construction will be better understood from the following description of a specific embodiment thereof in conjunction with the accompanying drawings of which:

FIG. 1 is a side elevation, partly in section, of a multiple hearth furnace embodying the improvement of our invention;

FIG. 2 is an enlarged sectional view of the upper end and the uppermost hearth of the multiple hearth furnace shown in FIG. 1;

FIG. 3 is an enlarged sectional view of the lower end 'and the lowermost hearth of the furnace shown in FIG. 1;

FIG. 4 is an enlarged view, partly in section, of one of the radially extending rabble arms embodying our invention;

FIG. 5 is a sectional view along line 55 of FIG. 4 showing the multiple layers of insulating and corrosion resistant refractory materials covering the metal core of the rabble arm;

FIG. 6 is a perspective view of the inner end of the metal core of one of the rabble arms embodying the present invention; and

FIG. 7 is a sectional view along line 77 of FIG. 6.

Detailed description For the purpose of illustration, the improved furnace construction of our invention will be described in conjunction with a multiple hearth furnace of the Herreshotf type that is adapted to carry out the decomposition of aluminum chloride hexahydrate and the calcination of the aluminum oxide product obtained thereby. Accordingly, as shown best in FIG. 1 of the drawing, the multiple hearth furnace embodying our invention comprises, in its major components, :a generally cylindrical refractory-lined furnace body 11 having a refractory-lined roof 12 and refractory-lined floor or bottom hearth 13, a plurality of vertically spaced annular hearths 14 to. disposed between the bottom hearth 13 and the roof-12, a rotatable center shaft 21 that extends upwardly through the central openings of the annular hearths, a plurality of radially extending rabble arm 22 positioned above each hearth and adapted to sweep the material being calcined across the several hearths progressively from hearth to hearth, the generally cylindrical furnace body 11 and the furnace structure associated therewith being mounted on vertical support members 24 and the foundation structure 25.

The refractory-lined wall of the furnace body 11 comprises an exterior metal shell 27 having an inner lining 28 of firebrick or the like, and it is provided with gas or fuel oil burner ports 29 and access doors 30 at each hearth level. As shown best in FIG. 2, the roof 12 of the furnace comprises an exterior metal member 32 having an inner lining 33 of firebrick or the like, and it is provided with a gas discharge opening or passageway 34 having an upward extension 35 and a feed inlet opening or passageway 36 having an upward extension 37 through which the hydrogen chloride-containing furnace gases and the aluminum chloride hexahydrate feed material are respectively discharged from and introduced into the furnace. As shown best in FIG. 3 the floor or bottom hearth 13 of the furnace comprises an exterior metal supporting member 39 having an inner lining 40 of firebrick, or the like, and it is provided with a calcine discharge conduit 41 through which the calcined alumina is withdrawn from the bottom hearth of the furnace and also with a secondary calcine discharge outlet 42 that is normally closed off by a plate 43. The vertically spaced annular hearths 14 to 20 disposed within the furnace are formed of firebrick or some other equivalent refractory material, each hearth being provided with a combined calcine drop hole and gas riser passageway 44 to 50, respectively, the aforementioned passageway being disposed alternately at the center and at the periphery of successive hearths.

As shown in FIGS. 1 and 3, the vertically disposed rotatable center shaft 21 comprises an internally cooled hollow metal core 51 that, within the confines of the furnace, comprises an inner metal tube 52 and an outer metal tube 53 which is completely covered with a unique multilayer combination of insulating and corrosion resistant refractory materials as hereinafter more full described. The center shaft 21 is rotatably mounted on a suitable thrust bearing 54 which, in turn, is mounted on the foundation structure 25. Drive means are provided for rotating the center shaft 21 which, in the embodiment of our apparatus shown in the drawing, comprise a bevel gear 55 and pinion gear 56 which are driven through a reduction gear 57 by a prime mover (not shown). As shown in FIG. 3, the interior of the inner metal tube 52 and the annular space between the tube 52 and the outer metal tube 53 comprise cooling fluid passageways for cooling air supplied by the blower 59. This cooling air is introduced into the interior of the metal tube 52 of the vertical center shaft 21 through the annular manifold 60 and the openings 61 formed in the lower end of the hollow metal core 51, and this air is discharged from the upper end of the metal core 51 by way of the annular space between the inner tube 52 and the outer tube 53.

As previously noted, within the confines of the furnace, that is, from the bottom hearth 13 to the roof 12 of the furnace, the outer tube 53 of the metal core 51 of the center shaft 21 is protected against attack by the extremely hot and corrosive furnace gases by a multi-layer combination of insulating and corrosion resistant refractory materials (shownin vertical section in FIGS. 1, 2, and 3), said multi-layer combination comprising an inner layer 62 of a relatively soft or unconsolidated insulating material that is immune to attack by hot hydrogen chloride gas and an outer layer 63 of a corrosion-resistant structural refractory material which has been molded and fired to obtain corrosion-resistant structural parts of the configuration or shape required. The inner layer 62 advantageously is made up largely of a particulate material such as micro-quartz or of a fibrous material such as silica wool, flame blown low alkali glass wool, aluminasilicate refractory wool or the like, and the outer layer 63 advantageously is made largely of firebrick (typically, about 50% SiO and 45% A1 0 or a high silica (at least about 90% SiO- or high alumina (at least about 90% A1 0 refractory composition, or the like. The combination of an inner layer 62 of particulate or fibrous refractory material and an outer layer 63 of structural refractory material, in conjunction with the internal cooling of the metal core 51 of the center shaft 21, serve to maintain the metal core at a temperature below that at which the core is attacked significantly by the hot hydrogen chloride-containing furnace gases, or below about 500 F., when the furnace gases are at a temperature of between about 1000 F. and 1800 F.

In like manner, the radially extending rabble arms 22 each com-prises an internally cooled hollow metal core 64 that is completely covered with a multi-layer combination of insulating and corrosion-resistant refractory materials equivalent to that which is employed in conjunction with the mtetal core of the center shaft 21. As shown best in FIGS. 4 through 7, the outer end of the hollow metal core 64 is closed off by an end wall 65 and is provided with two centrally disposed longitudinal partitions 66 which extend from the inner end of the metal core 64 to a point 66a spaced an appreciable distance inwardly from the end wall 65, whereby three cooling fluid passageways are provided within the interior of the metal core 64. The inner end of the metal core 64 is formed with a connector section 67 that is adapted to be mounted on and secured to the inner and outer tubes of the metal core 51 of the rotatable center shaft as shown in FIG. 3. The connector section 67 is provided with a cooling fluid inlet opening 68 that communicates with the interior of the inner metal tube 52 and with the innermost of the three cooling fluid passageways of the metal core 64. The connector section 67 is also provided with two cooling fluid discharge openings 69 which communicate with the two outermost of the three cooling fluid passageways of the metal core 64 and with the annular space between the inner tube 52 and the outer metal tube 53. As previously noted, the interior of the inner metal tube 52 contains an upwardly flowing stream of cooling air introduced thereinto by the blower 59, and a portion of this cooling fluid is introduced into the interior of the hollow metal core 64 of the rabble arm 22 through the cooling fluid inlet opening 68. The cooling fluid thus introduced into the interior of the metalcore 64 circulates through the three cooling fluid passageways as indicated by the arrows in FIG. 7 and then is returned through the discharge openings 69 to the annular space between the inner tube 52 and the outer tube 53 of the metal core 51.

The multi-layer combination of corrosion-resistant insulating and refractory materials covering the internally cooled metal core 64 of each rabble arm 22 comprises an inner layer 70 formed of a relatively soft unconsolidated refractory material'that is immune to attack by hot hydrogen chloride-containing gases and an outer layer 71 formed of a preformed (that is, molded and fired) structural refractory material that also is resistant to attack by the highly corrosive furnace gases. The refractory mateial from which the inner layer 70 is formed advantageously is a fibrous material such as silica wool, a flame blown low alkali glass wool or an alumino-silicate refractory wool, and the structural refractory material from which the outer layer 71 is formed advantageously is firebrick or a high silica or alumina based refractory com-position. The aggregate thickness of the inner layer 70 of fibrous insulating material and the outer layer 71 of structural refractory material is such that, in conjunction with the internal cooling of the metal core 64 of the rabble arm, the metal core is maintained at a temperature below that at which the metal core is attacked significantly by the hydrogen chloride-containing furnace gases, or at a temperature of below about 500 F., when the furnace gases are at a temperature of between about 1000 F. and 1800 F. In this connection we have found that the inner layer 70 may range from about /2 to 1 /2 inch in thickness and that the outer layer 71 may range from about /2 to inch in thickness, depending on the location in the furnace.

The outer layer 71 of preformed structural refractory material is made up of two sections or members, the upper section 71a comprising a generally semi-cylindrical member (which is shown in longitudinal section in FIG. 4 of the drawing) which rests on the underlying layer 70 of fibrous refractory material, and the lower section 71b comprising a generally planar member which is provided with integrally formed dependent rabble teeth 72 that are adapted to sweep the material being calcined across the surface of the hearth 'with which the rabble arm 22 is associated. The generally semi-cylindrical upper section 71a is provided with a pair of dependent transverse chord elements 73 which are integrally formed on the upper inside surface of the upper section 71a, the transverse chord elements 73 engaging .a pair of upstanding lugs 74 integrally formed on the upper outside surface of the metal core 64 so as to prevent longitudinal movement of the upper section 71a with respect to the metal core 64. The generally planar lower section 71b is provided with a pair of inwardly turned longitudinal lip members 75 which rest on and are supported by longitudinal flange members 76 formed along the bottom longitudinal edges of the hollow metal core 64.

The unique two section construction of the outer layer 71 of structural refractory material and the manner in which each section is mounted on or is supported by the metal core 64 of the rabble arm 22 greatly facilitate replacement or adjustment of either or both sections of the outer layer 71. Moreover, the lower section 71b of the outer layer 71 may be fabricated in one piece as indicated in the drawing or, advantageously, it may comprise a number of individual pieces or segments which when assembled on the flange member 64 form a complete lower section 71b as shown. This feature of our improved construction is particularly important in view of the fact that the rabble teeth 72 are integrally formed elements of the lower section 71b of said outer layer 71 of refractory material. It will also be noted that our improved construction insures that the inner layer 70 of relatively soft refractory material will be subjected to compessive forces only. This is importont because this relatively soft and preferably fibrous refractory material would be unable to withstand the tensile and shear forces generated by the tangential forces acting on the dependent rabble teeth 72 to which the layer 70 might otherwise be subjected.

As noted, the downwardly extending, integrally formed rabble teeth 72 are adapted to sweep the material being calcined across the hearth with which the rabble teeth are associated progressively from hearth to hearth, and to this end the various rabble teeth are disposed at an angle with respect to the longitudinal axis of the rabble arm from which they depend. However, as the material being calcined must alternately be swept inwardly toward the central drop hole (or outwardly toward the peripheral drop hole, as the case may be), of one hearth and then outwardly toward the peripheral drop hole (or inwardly toward the central d-rop hole, in the latter case), of the hearth therebelow, and as the vertical center shaft 21 and the radially extending rabble arms 22 associated therewith all rotate in the same direction, the rabble teeth 72 associated with each one of the hearths must be disposed at an angle or pitch opposite to that of the rabble teeth of the immediately adjacent hearths. Moreover, while it is obvious that the number of hearths and the number of rabble arms associated with each hearth are not critical, we have found that acceptableresults are obtained when eight hearths are employed and when the uppermost hearth 20 is served by four radial rabble arms while the remaining hearths are served by two radial rabble arms apiece.

As the temperature of the furnace gas is somewhat lower in the upper portion of the furnace than in the lower portion of the furnace, and as the furnace gas becomes richer in hydrogen chloride and water vapor as it progresses from the lower to the upper hearths of the funace, there is some tendency for hydrochloric acid to condense on the inner surface of the metal shell 27 adjacent the upper hearths of the furnace. Therefore, in order to prevent the hydrogen chloride and water vapor from condensing on the inner surface of the exterior metal shell 27 of the furnace, a layer of insulation 78 is applied to the outer surface of the metal shell 27 in order to maintain the temperature of the metal above the dew point of hydrochloric acid, or above about 150 F. It has been found in practice that the layer 78 of insulating material advantageously has an insulating value equivalent to an 85% magnesia blanket wrapped with chicken wire mesh and troweled with A inch 85% magnesia cement, the aggregate thickness of the layer 78 being about 2 inches adjacent hearths 20 and 19 and about 1 inch adjacent hearths 18, 17, 16, and of the furnace. The lower two hearths 14 and 13 do not require external insulation as the relatively high temperature of the furnace gases thereat and the relatively low hydrogen chloride content of this gas preclude the condensation of any significant quantity of hydrochloric acid on the inner surface of the metal shell 27 adjacent these hearths.

In order to prevent the escape of corrosive furnace gases into the atmosphere at the point where the rotatable center shaft 21 extends through the roof 12 and the bottom hearth 13 of the furnace, the center shaft is provided with upper and lower gas-tight dry seals 80 and 81 that are associated with the roof and the bottom hearth, respectively. As shown in FIGS. 1, 2, and 3, each of the said dry seals comprises an annular well, partly filled with dry sand, that is secured to one of the mutually movable parts (that is, either to the rotatable center shaft or to the stationary furnace shell) and a dependent cylindrical skirt member that is secured to the other of the mutually movable parts and that extends into the body of dry sand contained in the aforesaid annular well. Thus, as shown best in FIG. 2, the upper dry seal 80 comprises a stationary annular well that is partly filled with dry sand 82 and a rotatable cylindrical skirt member that extends into the sand 82. The stationary annular well comprises an outer cylindrical member 83 secured to the furnace shell, a flange member 84 secured to the outer cylindrical member 83 and an inner cylindrical member 85 secured to the flange member 84. The rotatable cylindrical skirt comprises a flange member 86 secured to the metal core 52 of the rotatable center shaft 21 and a dependent cylindrical member 87 that is secured to the flange member 86. Similarly, as shown best in FIG. 3, the lower dry seal member 81 comprises a rotatable annular well partly filled with dry sand 88 and a stationary cylindrical skirt member that extends into the sand 88. The rotatable annular well comprises an inner cylindrical member 89 secured to metal core of the center shaft 21, a flange member 90 secured to the inner cylindrical member 89 and an outer cylindrical member 91 secured to the flange member 90. The stationary cylindrical skirt comprises simply a dependent cylindrical member 92 secured to the metal supporting member 39 of the bottom hearth 13.

The inventive concept underlying our improvement in the high temperature calcination of solid materials resides in the measures we have devised to protect the metal parts of the furnace against attack by such hot, highly corrosive gases as hydrogen chloride, chlorine, and the like. As described herein, these measures involve providing means for cooling and insulating the metal parts of the furnace exposed to the hot corrosive furnace gases that are capable of maintaining these metal parts at a temperature above the dew point of hydrochloric acid and below about 500 F. when the furnace gases are at a temperature of between about 1000 F. and 1800 F. or higher. The means we have devised for cooling and, especially, for insulating the metal parts of the furnace may be employed in conjunction with a variety of types of furnace construction and they are not limited to the multiple hearth furnace referred to in the foregoing detailed description of the invention. Moreover, the measure devised to protect the metal parts of the furnace against attack by hydrogen chloride or chlorine gas are equally effective to protect these metal parts against corrosive attack by such gases as oxygen, water vapor, carbon dioxide, carbon monoxide, sulfur dioxide, and the like.

The following specific example is illustrative but is not limitative of the practice of our invention.

A multiple hearth furnace of the Herresholf type substantially as shown in the accompanying drawings was employed to decompose aluminum chloride hexahydrate and to calcine the resulting aluminum oxide product. The outside diameter of the metal furnace shell was 8 ft. 6 in., and the interior of the shell was lined with insulation and firebrick nominally 9 in. thick. The furnace was provided with eight hearths. The exterior of the furnace shell adjacent the two upper hearths was insulated with an magnesia blanket 2 in. thick, and adjacent the four middle hearths the furnace shell was insulated with an 85% magnesia blanket 1 in. thick. The exterior of the furnace shell adjacent the two bottom hearths was uninsulated. Within the confines of the furnace (that is, between the bottom hearth and the roof of the furnace) the internally cooled rotatable metal center shaft was insulated with an inner layer of micro-quartz approximately 3 in. thick and an outer layer of a molded high alumina refractory about 1 in. thick. The internally cooled metal core of each radial rabble arm was similarly insulated with an inner layer of a blanket of fibrous silica wool and an outer layer of a molded high alumina refractory. The outer layer of refractory material was formed in two sections, the lower section being provided with integrally formed rabble teeth as previously described. Gas burners were provided at each hearth level except the top hearth. Aluminum chloride hexahydrate was introduced or fed onto the top hearth through the feed inlet opening and hot furnace gases containing between 20 and 30% HCl were withdrawn through the gas discharge opening formed in the furnace roof. Substantially pure pot line grade aluminum oxide was withdrawn through the calcine discharge opening formed in the bottom hearth of the furnace. The temperature of the furnace gases. at the top hearth was be! tween about 500 F, to 800 F., and the temperature of the furnace gases at the next five hearths therebelow ranged between about 1000 F. to 1700 F. The temperature of the furnace gases at the last two hearths ranged from about 1800 F. to above 2200 F. Thermal decomposition of aluminum chloride was substantially complete by the time the solid material was discharged from the sixth hearth, the aluminum oxide product being heated from about 700 F. to about 1700 F. at the seventh (or next to the bottom hearth and from about 1700 F. to about 2100 F. at the bottom hearth. The temperature-of the inner surface of. the furnace shell and the outersurfaces of the metal parts of the rotatable center shaft and radial rabble arms was between about 300 F. and 400 F. at the first six hearths of the furnace. The temperature of these metal surfaces at the last two hearths ranged between about 400 F. and 700 F., but the hydro- 9 gen chloride content of the furnace gases-at. these hearths was negligible.

From the foregoing description-of the improvements in furnace construction of our invention it will be seen that we have made an'important contribution to the art to which our invention relates.

We claim:

1. In apparatus for heating solid materials in the presence of hot corrosive gases in a furnace having internal metal parts exposed to said furnace gases, the improvement which comprises protecting the internal metal parts of the furnace against attack by said corrosive furnace gases by insulating said metal parts with an inner layer of a relatively soft insulating material that is immune to attack by hydrogen chloride-containing gases at a temperature of at least 1800 F. and with an outer layer of a preformed structural refractory material that also is resistant to attack by hot hydrogen chloride gas and that protects the underlying layer of insulating material from physical attrition, and by internally cooling said metal parts exposed to said corrosive gases so that said metal parts are maintained at a temperature above the dew point of hydrochloric acid and below about 500 F. when said furnace gases are at a temperature of between about 1000 F. and 1800 F.

2. The apparatus according to claim 1 in which the relatively unconsolidated soft insulating material is selected from the group consisting of micro-quartz, silica wool, low alkali glass wool, and alumino-silicate refractory wool.

3. The apparatus according to claim 1 in which the structural refractory material is selected from the group consisting of firebrick, a silica based refractory containing at least about 90% SiO and an alumina based refractory containing at least about 90% A1 4. The apparatus according to claim 1 in which the inner layer of relatively unconsolidated soft refractory material is from about /2 to 3 inch in thickness, and in which the outer layer of structural refractory material is from about /2 to 1 /2 inch in thickness.

5. In apparatus for heating solid materials in the presence of hot gases comprising a refractory-lined metal furnace shell, at least one annular hearth disposed within the furnace, a vertically disposed rotatable center shaft which extends upwardly through the central opening of the annular hearth, and a plurality of radially extending rabble arms secured at their inner ends to the rotatable center shaft above the annular hearth,'said rabble arms having rabble teeth depending therefrom which are adapted to sweep the solid material being treated across the annular hearth to a discharge opening formed therein, the improvement which comprises protecting the furnace against attack by corrosive furnace gases by insulating and cooling the metal parts of the furnace exposed to said corrosive gases so that said metal parts are maintained at a temperature above the dew point of hydrochloric acid and below about 500 F. when said furnace gases are at a temperature of between about 1000 F. and 1800 F.,

said rotatable center shaft comprising a hollow metal core provided with cooling fluid passageways for the circulation of a cooling fluid therethrough, said metal core being covered with an inner layer of a relatively soft insulating material that is immune to attack by hydrogen chloride-containing gases at a temperature of at least 1800 F. and with an outer layer of a preformed structural refractory material that also is resistant to attack by hot hydrogen chloride gas and that protects the underlying layer of insulating material from physical attrition,

each of said radial rabble arms comprising an essentially hollow metal core provided with cooling fluid passageways which communicate with the cooling fluid passageways in the hollow metal core of the rotatable center shaft, the metal core of said rabble arms covered with an inner layer of relatively soft fibrous insulating material that is immune to attack by hydrogen chloride-containing gases at a temperature of at least about 1800 F. and with an outer layer of preformed structural refractory material that also is resistant to attack by hot hydrogen chloride gas, said outer layer comprising a generally semi-cylindrical upper section that fits over the inner layer of fibrous insulating material and a generally planar lower section having integrally formed rabble teeth depending therefrom, the inner layer of relatively soft insulating material disposed between the metal core and the outer layer of preformed refractofly material being subjected to compressive forces 0 y.

6. The apparatus according to claim 5 in which the furnace is provided with a plurality of vertically spaced annular hearths, the rabble arms and dependent rabble teeth associated with each hearth being adapted to sweep the solid material being treated across each annular hearth progressively from hearth to hearth, and in which the exterior of the metal shell of the furnace adjacent the upper hearths is covered with a layer of insulating material having an insulating value equivalent to an magnesia blanket about two inches thick and adjacent the intermediate hearths is covered with a layer of insulating material having an insulating value equivalent to an 85% magnesia blanket about one inch thick whereby the temperature of the metal shell is maintained above the dew point of hydrochloric acid during the aforesaid heatmg operation.

7. The apparatus according to claim 5 in which the relatively unconsolidated soft insulating material is selected from the group consisting of micro-quartz, silica wool, low alkali glass wool, and alumino-silicate refractory wool.

8. The apparatus according to claim 5 in which the structural refractory material is selected from the group consisting of firebrick, a silica based refractory containing at least about SiO and an alumina based refractory containing at least about 90% A1 0 9. The apparatus according to claim 5 in which the inner layer of relatively unconsolidated soft refractory material covering the metal core of the rotatable center shaft and the radial rabble arms is from about /2 to 3 inch in thickness, and in which the outer layer of structural refractory material covering the rotatable center shaft and the radial rabble arms is from about /2 to 1 /2 inch in thickness.

10. The apparatus according to claim 5 in which the generally semi-cylindrical upper sections of the outer layer of structural refractory material that covers the radial rabble arms are each provided with a pair of dependent transverse chord elements which are integrally formed on the upper inside surface of said upper section, the transverse chord elements engaging a pair of upstanding lugs integrally formed on the upper outside surface of the metal core whereby longitudinal movement of the upper section with respect to the metal core is prevented.

11. The apparatus according to claim 5 in which the generally planar lower sections of the outer layer of structural refractory material that covers the radial rabble arms are each provided with a pair of inwardly turned longitudinal lip members which rest on and are supported by longitudinal flange members formed along the bottom longitudinal edges of the hollow metal core of each rabble arm.

12. In the process wherein a metal chloride is heated in the presence of water to effect the thermal decomposition of said metal chloride and the formation of hydrogen chloride gas and the corresponding metal oxide, followed by the calcination of said metal oxide at a temperature of at least about 1600? F., the improvement which comprises protecting the metal parts of the furnace in which said decomposition and calcination operations are carried out against attack by the h'o't'hydrogeh chloride-containing furnace gase's'by insulatingsaid metal parts with an inner layer of a relatively soft insulating 'material that is immune to attack by hydrogen chloride-containing gases at a temperature of at least 1800 F. and with an outer layer of a preformed structural refractory material that also is resistant to attack by hot hydrogen chloride gas and that protects the underlying layer of insulating material from physical attrition, and by internally cooling the thus ins'ulated metal parts so that'said metal parts are maintained at a temperature above the dew point of hydrochloric acid and below about 500 F. when said furnace gases are at a temperature of between about 1000 F. and 1800 F.

13. The process according to claim 12 in which the relatively unconsolidated soft insulating material is selected from the group consisting of micro-quartz, silica wool, low alkali glass wool, and alumino-silicate refractory wool.

14. The process according to claim '12 in which the structural refractory material is selected from the group consisting of firebrick, a silica based refractory containing at least about 90% SiO and an alumina based refractory containing at least about 90% A1 0 15. The process according to claim 12in which the metal chloride is aluminum chloride hexahydrate.

References Cited UNITED STATES PATENTS 4/1943 Rowen 263-26 JOHN J. CAMBY, Acting Primary Examiner. 

